In this paper ancillary services performed by Distributed Generation (DG) sources connected to the grid are discussed. Wind energy is analyzed as case study of renewable DG source. Variable speed wind turbines are connected to the grid by power electronics converters, which are typically controlled in order to transfer to the grid the whole captured active power from the wind. Moreover a proper control of the power converters makes the DG able to perform some ancillary services. In this paper Point of Common Coupling (PCC) voltage amplitude regulation, reactive power compensation and active parallel filtering are analyzed. Converter sizing is discussed to be the limiting factor in performing the considered ancillary services.

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A preliminary study for the development of a frequency domain methodology aimed to assess the Light Flicker "penetration" in the electrical networks due to the interharmonic distortion produced by wind turbines is conducted. Starting from the numerical models of two wind turbine schemes, the former fixed and the latter variable speed, simplified analytical formulas already proposed for the P st evaluation based on voltage spectral analysis are tested.